This invention relates to silicone-based surfactants, and in particular silicone-based surfactants useful as chelating agents.
A wide variety of applications require control of the interfacial properties between immiscible components, such as water-in-oil emulsions or oil-in-water emulsions. Generally, to obtain good performance it is necessary to stabilize the interface between the two immiscible components. One simple example is the use of coupling agents to modify silica surfaces so that silica may be used to reinforce organic polymers, with which it is otherwise incompatible. Another example is the use of surfactants to stabilize oils in water, such as in cleaning and conditioning applications.
Silicones are among the most surface-active materials (xe2x80x9csurfactantsxe2x80x9d) known. They diffuse rapidly to interfaces and readily spread. Spreading of the silicone may be facilitated by the incorporation of polar groups on the silicone backbone. Some of the most effective spreading compounds, particularly at solid/liquid/air surfaces, are the so-called xe2x80x9csuperwettersxe2x80x9d made by manufacturers including Crompton Corp. and Dow Corning. The general structure of these superwetters is (Me3SiO)2SiMe(CH2)3(OCH2CH2)nOZ, where Z may be H, CH3, CH3COO, etc.
Liquid-liquid interfaces are generally stabilized with silicones bearing non-ionic hydrophilic groups. Common examples include derivatives of so-called silicone polyols; that is silicones containing polyether sidechains. U.S. Pat. No. 5,707,613 to Hill teaches that these compounds are particularly useful water/silicone interfaces. Ionic silicone copolymers can also be used to stabilize such interfaces. U.S. Pat. No. 5,124,466 to Azechi et al. (Shin-Etsu) teaches that ammonium-modified silicone surfactants are useful in the stabilization of silicone emulsions in water.
The surface activity of silicones, whether cationic, zwitterionic or non-ionic, cannot be readily changed, although pH modifications may affect the behavior of some types of ammonium compounds. There are advantages in being able to change the surface activity of a surface active material so as to change the properties of systems in accordance with its particular use, for example, to flocculate emulsions on demand. For example, carboxylic acids and polymers derived from them (e.g., CARBOPOL(trademark) (available from BF Goodrich)) change their ability to swell water and to stabilize interfaces upon pH changes: bases convert neutral carboxylic acids to carboxylates. In this respect, silicones having a pH sensitivity, by virtue of amine or carboxylic acid groups, are known. U.S. Pat. No. 5,447,997 to Releigh et al. teaches silicones containing carboxylic acids whose surface properties change as a function of pH.
The properties of ionic surfactants may not only be changed by pH, but by the nature of the counterions. For example, carboxylates with monovalent counterions such as sodium swell well with water. In contrast, multivalent counterions in the same system, lead to ionic crosslinking and a reduction of swelling. At an interface, the surface activity of such materials are similarly affected by the nature of the counterion.
Multidentate ligands (or xe2x80x9cchelating agentsxe2x80x9d) bind metals very tightly. The classic example is EDTA (ethylenediaminetetraacetic acid). EDTA, normally in its calcium, disodium salt form, is frequently found in food products. Heavy metal ions coming into contact with the EDTA will complex with the amine and carboxylic acid groups, displacing the sodium/calcium ions. The binding efficiency of EDTA and its derivatives is known for many metals and their different oxidation states. Chelating agents are added to many different formulations for different purposes. They have also been bound to polymers. For example, chelating groups similar to those mentioned above are used as supports in affinity chromatography.
However, there still exists a need for silicones that are effective at chelating metal ions using complementary binding, whose properties may be controlled through the relative amounts and morphology of the hydrophilic and hydrophobic blocks, the chelating agent, the pH of the solution, the presence or absence of multivalent counterions, and the specific nature of the multivalent ions.
The present invention relates to silicone polymers useful as both surfactants and chelating agents. The polymers contain a hydrophobic component (the silicone polymer backbone) and a hydrophilic component. The hydrophilic component may act as a chelating agent; i.e. it will bind a variety of metals. The hydrophilic component may be hydrophilic prior to binding to a metal, or after binding to a metal.
The hydrophobic nature of the silicone is provided by organic radicals, such as methyl or other alkyl groups, modified alkyl groups such as fluoroalkyl groups, aryl groups, and related hydrophobic moieties, bound to the silicon atoms in the polymer. The hydrophilic component includes multiple ligands to cooperatively bind one or more metal centers. Examples of such ligands are well known in the art, and include hydrophilic groups such as carboxylic acids and their derivatives, amines, phosphines, alcohols, and unsaturated systems (multiple bonds) that are or are rendered hydrophilic upon complexation with a metal ion.
In one aspect, the present invention relates to a silicone polymer comprising a hydrophobic polysiloxane backbone and at least one metal binding site which is covalently bound to the hydrophobic polysiloxane backbone, the at least one metal binding site comprising at least two ligands which are optionally bound to a metal.
In one embodiment, at least one of the ligands is hydrophilic either before or after being bound to a metal. The ligand may include groups selected from functional alkyl groups bearing heteroatom-based ligands, functional aryl groups bearing heteroatom-based ligands, functional alkyl groups bearing heteroatom-based ligands where the ligands have exchangeable hydrogen atoms, functional aryl groups bearing heteroatom-based ligands where the ligands have exchangeable hydrogen atoms, functional alkyl groups having xcfx80-ligands, and functional aryl groups having xcfx80-ligands. Preferably, the metal binding sites include two or more carboxylic acids which may act as ligands.
The metal binding site may be covalently bonded to the silicone polymer backbone by a linker, which is at least as stable to hydrolysis as the siloxane linkage in the silicone polymer. The linker may be selected from single atoms including C, N, O, S, or P, or groups including amides, esters, thioesters, urethanes, ureas, alkyl or aryl groups.
The polymers of the invention may have molecular weights from about 500 to about 500,000 g/mol.
In one embodiment, the invention relates to a compound of the formula I:
T1(Q1)a(Q2)b(Q3)cT2xe2x80x83xe2x80x83(I)
where a, b and c are independently greater than or equal to 0; and where Q1, Q2 and Q3 are independently the same or different and are an internal siloxane group of the formula II: 
and, where R4 and R5, for each internal siloxane group, are the same or different, and where R4 and R5 are independently, H with the proviso that both R4 and R5 are not H on the same internal siloxane group, alkoxy, siloxy, alkyl, aryl, functional alkyl, functional aryl, a metal-binding site comprising at least two ligands optionally bound to a metal, or a group having an internal siloxane group of the formula III: 
where r is xe2x89xa70;
R7, and R8, are for each internal siloxane group of the formula III the same or different, and R6, R7, and R8 are independently, H with the proviso that not more than one of R6, R7, and R8 on each internal siloxane group is H, alkoxy, siloxy, alkyl, aryl, functional alkyl, functional aryl, or a metal binding site comprising at least two ligands optionally bound to a metal;
T1 is a group of the formula (IV): 
T2 is a group of the formula (V): 
xe2x80x83wherein R1, R2, R3, R9, R10, R11, are independently, H with the proviso that each silicon atom has no more than one H, alkoxy, siloxy, alkyl, aryl, functional alkyl, functional aryl, or a metal binding site comprising at least two ligands optionally bound to a metal;
with the proviso that at least one of R1 to R11 is a metal binding site comprising at least two ligands optionally bound to a metal, and with the proviso that the molecular weight of the compound is between 500 and 500,000 g/mol.